1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a nonvolatile semiconductor memory system.
2. Related Art
As one of nonvolatile semiconductor memory devices, NAND flash memory has been widely known. SSD (Solid State Drive) using plural NAND flash memories has been also widely known. The NAND flash memory (hereinafter, also “flash memory”) and the SSD are designed to operate using a single-standard supply power source to enable application of these devices to various devices by simplifying system designs.
There are also flash memories capable of using power sources of plural different voltages. In this case, different power sources are used for each of an I/O circuit and other memory elements. That is, substantially, these flash memories are also designed to operate using a single power source.
For example, when a 5 V power source is used for a flash memory that conforms to a 3.3 V power source standard, the 5 V needs to be once stepped down to 3.3 V.
However, the flash memory requires a voltage equal to or above 3.3 V (20 V, for example) to write data and to erase data. At a data reading time, a voltage equal to or above the 3.3 V power source standard (6 V, for example) needs to be applied to an unselected word line. Considering a voltage drop in wiring, a higher voltage than this (9 V, for example) is actually necessary. In this case, a high voltage boosted from the regulated voltage is required in the flash memory.
When the stepped-down low voltage is boosted in this way, a step-up factor of a boosting circuit becomes large. This increases current consumption of the boosting circuit, and also increases time required to write data, erase data, and read data.